1. Technical Field
The invention relates to conical refiners or disc-conical refiners for lignocellulosic materials, such as refiners used for producing mechanical pulp, thermomechanical pulp and a variety of chemi-thermomechanical pulps (collectively referred to as mechanical pulps and mechanical pulping processes).
2. Prior Art
Conical refiners, or conical zones of disc-conical refiners, are used in mechanical pulping processes. The raw cellulosic material, typically wood or other lignocellulosic material (collectively referred to as wood chips), is fed through the middle of one of the refiners discs and propelled outwards by a strong centrifugal force created by the rotation of a rotor disc. Refiner plates are mounted on each of the opposing faces of the refiner discs. The wood chips move between the opposing refiner plates in a generally radial direction to the outer perimeter of the plates and disc section when such a section exists (in disc-conical refiners). In conical refiners (or conical section of disc-conical refiners), the convex rotor element propels the wood chips into the concave stator element.
Steam is a major component of the feeding mechanism. Steam generated during refining displaces the wood chips through the conical zone.
In conical and disc-conical refiners, the refiner rotor conventionally operates at rotational speeds of 1500 to 2100 revolutions per minute (RPM). While the wood chips are between the refining elements, energy is transferred to the material via refiner plates attached to the rotor and stator elements.
The refiner plates generally feature a pattern of bars and grooves, as well as dams, which together provide a repeated compression and shear actions on the wood chips. The compression and shear actions acting on the material separates the lignocellulosic fibers out of the raw material, provides a certain amount of development or fibrillation of the material, and generates some amount of fiber cutting which is usually less desirable. The fiber separation and development is necessary for transforming the raw wood chips into a suitable board or paper making fiber component.
In the mechanical pulping process, a large amount of friction occurs, such as between the wood chips and the refiner plates. This friction reduces the energy efficiency of the process. It has been estimated that the efficiency of the energy applied in mechanical pulping is in the order of 10% (percent) to 15%.
Efforts to develop refiner plates which work at higher energy efficiency e.g., lower friction, have been achieved and typically involve reducing the operating gap between the discs. Conventional techniques for improving energy efficiencies typically involve design features on the front face of refiner plate segments that usually speed up the feed of wood chips across the refining zone(s) on the refiner plates. These techniques often result in reducing the thickness of the fibrous pad formed by the wood chips flowing between the refiner plates. When energy is applied by the refiner plates to a thinner fiber pad, the compression rate applied to the wood chips becomes greater for a given energy input and results in a more efficient energy usage in refining the wood chips.
Reducing the thickness of the fiber pad allows for smaller operating gaps, e.g., the clearance between the opposing refiner plates. Reducing the gap may result in an increase in cutting of the fibers of the wood chips, a reduction of the strength properties of the pulp produced by the discs, an increased wear rate of the refiner plates, and a reduction in the operating life of the refiner plates. The refiner plate operational life reduces exponentially as the operating gap is reduced.
The energy efficiency is believed to be greatest toward the periphery of the refiner discs, and in general, the same applies for both flat and conical refining zones. The relative velocities of refiner plates are greatest in the peripheral region of the plates. The refining bars on the refiner plates cross each other on opposing plates at a higher velocity in the peripheral regions of the refiner plates. The higher crossing velocity of the refining bars is believed to increase the refining efficiency in the peripheral region of the plates.
The wood fibers tend to flow quickly through the peripheral region of the conventional refiner plates, regardless of whether they are flat or conical in shape. The quickness of the fibers in the peripheral region is due to the effects of centrifugal forces and forces created by the forward flow of steam generated between the discs. The shortness of the retention period in the peripheral region limits the amount of work that can be done in that most efficient part of the refining surface.